A fuel gas is usually used as a fuel in a fuel cell. Specifically, hydrogen gas, methane gas and like hydrogen-containing gases, or methanol and like liquids are reacted with oxygen in the air to generate electric energy. Most of the waste product generated in this process is water; only a small amount of carbon dioxide, carbon monoxide and like toxic waste is generated. This method has therefore attracted public attention in recent years as an environmentally friendly energy-generation technique. Unlike engines and turbines, fuel cells are quiet and highly efficient, and intensive research and development has been conducted to find the practical uses thereof as a promising energy-generation technique. In some fuel cells, methanol and like liquid fuels may be used instead of a fuel gas, and a liquid oxidizing agent that contains hydrogen peroxide or the like may be used instead of air and like oxidation gases.
Fuel cells are usable in various fields, and practically used in fuel cell-powered vehicles, etc. Possible applications of fuel cells include their use as energy systems in facilities that need a considerable heat source and a large amount of electric power for air conditioning and hot water supply; in energy systems for general households; and as power sources for devices such as PDAs, cellular phones, laptop computers, etc.
FIG. 14 illustrates the principal of fuel cell electric power generation, schematically showing the principal structure and electrochemical reaction of the unit cell. FIG. 14 illustrates an example wherein methanol is used as a fuel. As shown in FIG. 14, in the fuel cell, a fuel electrode (anode) 101 and an oxygen electrode (cathode) 103 are disposed to face each other, and an electrolyte layer 102 lies between the fuel electrode 101 and the oxygen electrode 103.
As shown in Formula (1) below, the supplied methanol (CH3OH) reacts with water (H2O) on the fuel electrode 101 side so that hydrogen ions (H+) and electrons (e−) are dissociated therefrom and carbon dioxide (CO2) is generated.CH3OH+H2O→6H++6e−+CO2  (1)
Hydrogen ions can migrate through the electrolyte layer 102, but electrons cannot. Therefore, while hydrogen ions diffuse through the electrolyte layer 102 and migrate to the oxygen electrode 103, electrons move toward the oxygen electrode 103 via a circuit 104 that connects the fuel electrode 101 with the oxygen electrode 103 outside.
On the oxygen electrode 103, supplied oxygen gas (O2) reacts with the hydrogen ions that have migrated through the electrolyte layer, and the electrons that have traveled from the fuel electrode react as shown in Formula (2) below to generate water (H2O).6H++6e−+3/2O2→3H2O  (2)
When methanol and oxygen gas are continuously supplied, the reactions shown in Formulae (1) and (2) occur continuously, so that electrons continuously flow in the circuit 104. In other words, by supplying fuel fluid (CH3OH, H2, etc.) and oxidizing fluid (O2 or the like) in a continuous manner to the unit cell shown in FIG. 14, electric current flowing from the oxygen electrode 103 to the fuel electrode 101, i.e., electric power, can be generated.
There are several types of fuel cells, including molten carbonate fuel cells, solid polymer fuel cells, phosphoric acid fuel cells, solid oxide fuel cells, alkaline fuel cells and the like, depending on the types of the electrolytes used. Among these, the operational temperature in molten carbonate fuel cells and solid oxide fuel cells is relatively high, about 600-700° C. and about 800-1,000° C., respectively. The operation temperature in other types of fuel cells is generally not higher than about 200° C.
In fuel cells operating at a high temperature (high-temperature fuel cells), the reaction of Formula (1) proceeds at the fuel electrode using heat energy. Increasing the efficiency of the above-mentioned reactions at the fuel electrode and oxygen electrode—in other words, increasing the speed of the reaction at each electrode—is an important object for fuel cells operating at a low temperature (low-temperature fuel cells). In order to accelerate the reactions of Formulae (1) and (2), a catalyst, usually platinum, is used in the fuel electrode 101 and oxygen electrode 103. Therefore, platinum plays a very important role in low-temperature fuel cells.
Examples of usable catalysts other than platinum include iridium, palladium, rhodium, ruthenium, and alloys of at least two of the aforementioned metals other than platinum; alloys of platinum and the aforementioned metals; titanium oxides, etc. However, because platinum is superior to the other catalysts, it is the most widely used catalyst for fuel cells.
Porous carbon electrodes are often used in fuel cells to allow fuel fluid or oxygen-containing fluid to pass through the fuel electrode 101 and the oxygen electrode 103, and accelerate the reactions of Formula (1) or Formula (2) at these electrodes. Furthermore, in low-temperature fuel cells, fine powders of catalysts such as platinum, etc., are supported on the inner surfaces of pores in the porous electrode. As described above, in order to accelerate the reactions of Formula (1) and Formula (2), catalysts, in particular platinum, which has an excellent effect, are essential.
However, platinum is a very expensive noble metal; this is one of the main reasons why fuel cells are expensive. Platinum easily bonds to CO gas, and therefore CO poisoning may occur due to CO gas in the fuel fluid, CO gas generated by the oxidation reaction at the fuel electrode, etc. When platinum is poisoned by CO, its catalytic ability greatly decreases.
Methanol, hydrogen gas, and methane gas are often used in fuel cells. Methanol, hydrogen gas and methane gas—in particular hydrogen gas and methane gas—usually contain a small amount of CO gas, because they are obtained using hydrocarbons of natural gas as a raw material. In fuel cells that use methanol, CO is formed during the oxidation of the methanol. This transitive variety of CO is highly stable as it is adsorbed on the surface of platinum, etc. Therefore, when platinum is used as a catalyst and methanol, hydrogen gas or the like is used as a fuel, CO poisoning of the platinum is inevitable. It is possible to use catalysts other than platinum, but such catalysts are inferior to platinum in their catalytic effects, resulting in slower reactions at the fuel electrode and oxygen electrode.
It is believed that by adding ruthenium or the like to platinum in a methanol fuel cell, CO poisoning of platinum can be prevented to some degree. This is because ruthenium apparently accelerates the oxidation of H2O so as to generate hydroxyl ions and oxidize CO to CO2. However, because it is still impossible to satisfactorily maintain the catalytic effects of platinum, the problem of CO-poisoned platinum has yet to be practically resolved.
Therefore, when platinum is used as a catalyst and methanol or hydrogen gas is used as a fuel fluid, the CO poisoning of platinum is unavoidable. However, catalysts other than platinum are inferior to platinum in their catalytic effects. Therefore, in the currently available fuel cell units, the reactions at the fuel electrode and the oxygen electrode are slow.
In order to solve the above-mentioned problem, fuel cells that do not use platinum as a catalyst have been proposed (for example, Patent Document 1). The fuel cell disclosed in Patent Document 1 comprises a fuel electrode, an oxygen electrode and an electrolyte layer lying between the fuel electrode and the oxygen electrode, wherein the fuel electrode is an III-IV compound semiconductor doped with p-type impurities. In this fuel cell, a reaction takes place wherein hydrogen gas is decomposed to hydrogen radicals, and the hydrogen radicals are dissociated into hydrogen ions and electrons at the fuel electrode. It is assumed that because this reaction proceeds smoothly, platinum is unnecessary. In other words, a p-type impurity-doped compound semiconductor probably functions as a catalyst for dissociating hydrogen gas into hydrogen ions and electrons.
A fuel cell using a semiconductor as its electrode, wherein the structure of a pn junction-type semiconductor is applied (for example, Patent Document 2), has also been proposed. The fuel cell disclosed in Patent Document 2 is a single chamber-type fuel cell, wherein the whole fuel cell is formed in a mixed gas atmosphere of a fuel gas and an oxygen-containing gas. In this respect, the fuel cell of Patent Document 2 is different from the double chamber-type fuel cell disclosed, for example, in Patent Document 1. The fuel cell of Patent Document 2 comprises a p-type semiconductor layer whose carriers are holes, an n-type semiconductor layer whose carriers are electrons, and a pn-mixture layer between the p-type semiconductor layer and the n-type semiconductor layer, wherein all of the layers are porous to such an extent that the mixed gas can pass therethrough.
The electric power generation mechanism of this fuel cell is probably as follows.
In the vicinity of the depletion layer (the pn junction), which is sandwiched between the p-type semiconductor and the n-type semiconductor, an oxygen gas is adsorbed on the surface of the p-type semiconductor and then polarized. At the same time, a hydrogen gas is adsorbed on the surface of the n-type semiconductor and then polarized. A positive charge is generated on the surface of the p-type semiconductor, and a negative charge is generated on the surface of the n-type semiconductor. In a series of processes, the adsorbed hydrogen ions (H+) and oxygen ions (O2−) are reacted, water (H2O) is generated, electrons in the valence band in the p-type semiconductor near the depletion layer (the pn junction) are excited, and holes are formed in the valence band. In the formed electron-hole pairs, electrons migrate to the n-type semiconductor and holes migrate to the p-type semiconductor. By this mechanism, a potential difference is generated between the p-type semiconductor (negative pole) and the n-type semiconductor (positive pole), and the potential difference can be output as electric power.
The fuel cells disclosed in Patent Documents 1 and 2, which use semiconductors, do not employ a catalyst. In order to achieve the reactions shown in Formula (1) and Formula (2) in an efficient manner in the fuel electrode and the oxygen electrode, the use of a catalyst, in particular platinum, is desirable. However, as described above, the amount of reaction in the fuel electrode and the oxygen electrode that can be promoted is an important object. Particularly when platinum is used as the catalyst, the problem of how to prevent the CO poisoning of platinum is important. However, effective means for accelerating the reaction speed and preventing the CO poisoning of platinum, when platinum is used as a catalyst, have yet to be developed. Therefore, there is demand for the further improvement of the catalytic activity of platinum.
Furthermore, known methanol fuel cells have a serious problem, known as “methanol crossover”. Methanol crossover is a phenomenon wherein the methanol supplied to the fuel electrode moves to the oxygen electrode after passing through an interlayer such as an electrolyte layer. The reaction at the oxygen electrode caused by the migrated methanol negates the electric power generation effect.
In other words, methanol in conventional fuel cells tends to cross over from the fuel electrode, through the electrolyte and to the oxygen electrode, where it reacts with oxygen and liberates heat without producing electricity, leading to a loss of methanol and a reduction in fuel cell voltage. It has been shown that losses of 100 mV to 140 mV at a given current density occur at the cathode.
Patent Documents 3 and 4 disclose methods for reducing methanol crossover; however, satisfactorily practical effects have not been obtained, and the problem of methanol crossover has therefore not been completely resolved.
Yet another drawback of methanol fuel cells is that, unlike in hydrogen fuel cells, the anode needs to be activated in order to increase its potential. This leads to the need for high catalytic loading at the electrodes to achieve considerable reaction speed. The greater the catalyst amount, the higher the cost of the fuel cell. Therefore, some attempt should be made to reduce the cost.
In conventional fuel cells employing solid polymer electrolytes, the catalyst deposited on the two sides of solid electrolytes is supported by, for example, carbon powders. This leads to an inefficient use of the catalyst. Additionally, the surface area (the two-dimensional surface area) is limited, and this makes it difficult to miniaturize the direct methanol fuel cell for low power application. There is thus a need to establish the efficient utilization of the catalyst by promoting reaction speed, so that the electric power generation efficiency can be increased. One example of such a method requires the increase of the surface area of the electrodes and the reduction of the catalytic loading at the electrodes.
In addition, there is demand for small size fuel cells for use in such devices as cellular phones, laptop computers and PDAs. In order to meet this demand, it is necessary to reduce the size of the electrode by accelerating the reaction speed at the oxygen electrode and the fuel electrode.
However, miniaturization of fuel cells that operate at low temperatures cannot be achieved unless several problems can be solved.    Patent Document 1    Japanese Unexamined Patent Publication No. 2004-319250    Patent Document 2    Japanese Unexamined Patent Publication No. 2004-199877    Patent Document 3    U.S. Pat. No. 5,599,638    Patent Document 4    U.S. Pat. No. 5,919,583